In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of radio frequency (RF) current through the gas. One class of HID lamps comprises electrodeless lamps which generate an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In particular, the lamp fill, or discharge plasma, is excited by RF current in an excitation coil surrounding an arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
For efficient lamp operation, the excitation coil must not only have satisfactory coupling to the discharge plasma, but must also have low resistance and small size. A practical coil configuration avoids as much light blockage by the coil as possible and hence maximizes light output. One such coil configuration is described in commonly assigned U.S. Pat. No. 4,812,702 of J.M. Anderson, issued Mar. 14, 1989, which patent is hereby incorporated by reference. The excitation coil of the Anderson patent has at least one turn of a conductor arranged generally upon the surface of a torus having a substantially rhomboid or V-shaped cross section on either side of a coil center line. Another exemplary coil configuration is described in commonly assigned, U.S. Pat. No. 4,894,591, of H.L. Witting, issued Jan. 16, 1990 which is hereby incorporated by reference. The Witting application describes an inverted excitation coil comprising first and second solenoidally-wound coil portions, each being disposed upon the surface of an imaginary cone having its vertex situated within the arc tube or within the volume of the other coil portion.
During operation of an HID lamp, as the temperature of the excitation coil increases, coil resistance increases, thereby resulting in higher coil losses. Hence, to increase coil efficiency, the excitation coil of an HID lamp is typically coupled to a heat sink for removing excess heat from the excitation coil during lamp operation. Such a heat sink may comprise, for example, heat radiating fins coupled to the ballast used to provide radio frequency (RF) power to the lamp, as described in commonly assigned U.S. Pat. No. 4,910,439 of S.A. El-Hamamsy and J.M. Anderson, issued Mar. 20, 1990 which patent is hereby incorporated by reference.
Although the hereinabove described HID lamp excitation coil configurations are suitable for many lighting applications, it is desirable to provide an excitation coil exhibiting even higher efficiency, e.g. in excess of 90%, while providing efficient heat dissipation from the coil and causing minimal light blockage from the lamp.